The present invention relates generally to coatings for application to recording sheets. More specifically, the present invention relates to an improved coating for ink jet recording sheets.
In the field of printing, particularly ink jet printing, it has been well-known to employ one or more ink jet assemblies connected to a source of ink. Each ink jet is electromagnetically energized to emit uniform droplets of ink as a continuous stream or as individual droplets. The droplets are directed onto the surface of a moving web and controlled to form printed characters. In view of this technique, the quality of printing, using the ink jet recording process, is highly dependent on jet operation and the properties of the ink and particularly the surface characteristics of the recording paper and the type of coatings applied thereon. The ink must be capable of forming stable ink droplets under pressure and must readily emerge from the ink jet. Aqueous inks containing a water soluble dye and a humectant, to prevent ink from drying out in the jet assembly, have been found to be desirable. However, the absorption of these inks by the recording sheet has been problematic, particularly in the area of multi-color printing where two or more ink drops may overlap on the surface of the recording sheet.
To ensure good image quality, the recording sheet must absorb the ink rapidly and at the same time insolubilize the ink dye on the sheet surface. Good absorption reduces the ability of the ink to transfer to sheet handling rollers and other printer machine parts while insolubility of the ink dye ensures high optical density. Unfortunately, papers with high absorbency draw the ink deeply into the paper which sacrifices optical density. Moreover, such recording sheets suffer from feathering and poor acuity. In addition, high quality papers with low absorbency suffer from set-off because the ink is not absorbed rapidly and smearing often occurs.
Due to the advent of high quality, low cost color ink jet printers and advancements in commercial papermaking, there is a demand for high color density and clear color tone resolutions. Paper for recording sheets used in such ink jet printing must rapidly absorb the ink and must be free from the flow-out of ink and from the danger of staining even if the paper is touched just after printing. Further, the diffusion of ink laterally on the recording sheet must be prevented in order to achieve high resolution without blurring. Thus, for obtaining color images, having good color density and resolution with good absorbing property and water fastness as well as optical brightness, a coated paper for ink jet recording must satisfy all of these requirements.
Various attempts have been made in the prior art to achieve a quality ink jet recording sheet which has each of these desired features. For example, U.S. Pat. No. 4,371,582, issued to Sugiyama et al. and incorporated herein by reference, discloses a recording sheet containing insoluble latex polymer which is designed to be used in ink jet recording with water soluble dyes to improve water fastness. The insoluble basic latex polymer is applied to the surface of the recording creating an impervious film that provides no absorbency for the water-soluble dyes. The Sugiyama patent suffers from offsetting for having applied water insoluble latex onto a highly water resistant paper creating an unpenetrable surface where the ink does not dry and smears. Also, this coating needs a cationic donor for water resistance.
In addition, U.S. Pat. No. 4,554,181, issued to Cousin et al. and incorporated herein by reference, discloses an ink jet recording sheet having a bicomponent cationic recording surface where a water soluble polyvalent metal salt is provided. The use of such a conductive resin serves as a humectant suffers from bad water resistance and is, therefore, subject to smearing.
Further, U.S. Pat. No. 4,576,867, issued to Miyamoto and incorporated herein by reference, discloses ink jet recording paper with a cationic resin attached to at least the surface thereof. Due to the lack of leveling and flow modifiers and dispersing agents, blotching and poor wettability will occur.
Conventional natural binders and cationic resins such as polyethylenimine, metal salts, or dicyandiamide formaldehyde condensate (carcinogenic) plyamines have not been sufficient to provide all the desired properties. When using natural water soluble binders and water soluble cationic resins, poor water resistance and inferior water fastness will result. Such cationic donors attach to the surface of the ink jet recording paper sheet and, as a result, facilitate smearing.
Therefore, a coating which allows ink jet printing with superior water resistance, water fastness, controlled wettability, and adjustable brightness is desired to address those needs not found in the prior art.